A large number of sensing approaches and transducer types have been developed for the determination of the level of liquids in open or closed vessels or containers. Not only is the knowledge of the level itself important, but other measurements can be inferred from the level. For example, if the container geometry and dimensions are additionally known, the volume of the liquid can be determined. If additionally, the density of the liquid is known, it mass can be calculated.
Liquid level is generally sensed by one of two methods: obtaining a discrete indication if a predetermined level has been reached (i.e. point sensing) or obtaining an analog representation of the level as it changes (i.e. continuous sensing). One commonly employed apparatus for continuously sensing the level of liquid, such as a fuel, in fuel storage tanks is the use of a float mechanism.
Another type of continuous level sensing is called dielectric variation sensing or capacitive liquid level sensing. Capacitive liquid level sensing is primarily used with relatively non-conductive liquids which play the role of a dielectric material between multiple electrodes which are used and are electrically connected as plates of a capacitor. The capacitance changes continuously as the vertical height of the liquid changes in the tank. However, it is often necessary to compensate for changes in the characteristics of the liquid during measurement.
The U.S. Patent to Lenker No. 3,555,904, discloses a fluid indicating system, including a thermocouple, which includes two elements, one of which is made from an electrical resistance material. A shorting mechanism is slidably interposed between the elements on one end of the thermocouple. The shorting mechanism is responsive to the quantity of fluid to position itself alone one element to thereby change the effective electrical resistance in series with an electrical measuring circuit.
The prior art discloses other various devices for measuring the level of a liquid in a container such as U.S. Pat. Nos. 3,644,691, 4,343,184, 4,454,761, 4,457,171, 4,552,090, 4,589,282, 4,601,165 and 4,630,478.
A force sensing resistor is an ink whose electrical resistance decreases with minimum applied force. Such ink is produced by Interlink Electronics of Santa Barbara, Calif. Such ink is also disclosed in U.S. Patents to Eventoff, Nos. 4,301,337, 4,314,228, 4,276,538 and 4,451,714.